The Weismann barrier, proposed by August Weismann, is the principle that hereditary information moves only from genes to body cells, and never in reverse. In more precise terminology, hereditary information moves only from germline cells to somatic cells (that is, soma to germline feedback is impossible). This does not refer to the central dogma of molecular biology, which states that no sequential information can travel from protein to DNA or RNA.
The Weismann barrier was of great importance in its day and among other influences it effectively banished certain Lamarckian concepts. It remains important, but has however required qualification in the light of modern understanding of horizontal gene transfer and some other genetic and histological developments. The use of this theory, commonly in the context of the germ plasm theory of the late 19th century, before the development of better-based and more sophisticated concepts of genetics in the early 20th century, is sometimes referred to as Weismannism. Some authors call Weismannist development (either preformistic or epigenetic) that in which there is a distinct germ line, differently of somatic embryogenesis. This type of development is correlated with the evolution of death of the somatic line.
Other evidence against Weismann's barrier is found in the immune system. A controversial theory of Edward J. Steele suggests that endogenous retroviruses carry new versions of V genes from soma cells in the immune system to the germ line cells. This theory is expounded in his book Lamarck's signature. Steele observes that the immune system needs to be able to evolve fast to match the evolutionary pressure (as the infective agents evolve very fast). He also observes that there are plenty of endogenous retroviruses in our genome and it seems likely that they have some purpose.
However, even if both of these possible exceptions turn out to be legitimate, the Weismann barrier loses only its absolute status, and may still remain impermeable in most cases. Without further examples, the penetration of the Weismann barrier is still very much an exception.
In plants, genetic changes in somatic lines can and do result in genetic changes in the germ lines, because the germ cells are produced by somatic cell lineages (vegetative meristems), which may be old enough (many years) to have accumulated multiple mutations since seed germination, some of them subject to natural selection.
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